Hey thinking about a gear swop. Let me tell you what I have.

351w RV cam, carb, Duraspark, AOD and currently 3.50 with Track Lok and 235/70/15 tires.

I'm thinking of going with a 275/60/15 tire in rear. Height is about the same at 28". I was also looking on jumping to 4.11s for a little more start up power.

I've checked charts on the Internet and not sure yet. Right now with my 3.50s on the freeway cruising at about 65 my rpms are about 1800.

I have a pretty good set up now and don't want to ruin that but improve it. Get a little more power off the line.

So looking for feedback on what you guys might be running and like.

Thanks for help
Kenny

 

Something is not right. According to the tire size, rpm calculator, you would be running at 2730 at 1:1. Theres no way that overdrive gear drops 900 rpm. Are you sure you have 3.50's now?

 

28 inch tires, 3.50 ratio, 65 mph:

1:1 = 2730, 24mph/1000 rpm
.67 = 1829, 36mph/1000 rpm

4.11 ratio:

1:1 = 3206, 20mph/1000 rpm
.67 = 2148, 30mph/1000 rpm

https://www.novak-adapt.com/knowledge/gearing/

There is no free lunch. 3.70s or 3.91s would split the difference but with less drama taking off.

You say you have a good setup now. If you are stop and go all the time with rare freeway, 4.11s are your friend. Mostly long haul, stay with what you have.

 

Tire Size /Width /Aspect Ratio /Rim Size /Overall Height

P235/70/15 /235 /70 /15 /27.95

P275/60/15 /275 /60 /15 /27.99

Tire sizes mentioned are pretty much identical.....

Tire Dia /RPM /Axle Ratio /Trans Ratio /MPH

27.95 /2735 /3.50 /1.00 /65.009

27.95 /1834 /3.50 /0.67 /65.064

27.95 /1975 /3.50 /0.67 /70.066

27.95 /2115 /3.50 /0.67 /75.033

27.99 /3207 /4.11 /1.00 /65.006

27.99 /2150 /4.11 /0.67 /65.046

27.99 /2315 /4.11 /0.67 /70.038

27.99 /2480 /4.11 /0.67 /75.029

even with the 4:11's, the highway RPM (@65 MPH) is not obcenely high, but unless you're on a secondary HWY, I'd expect you'd be doing more like 70....or more, which changes things a bunch.....

Only you know your typical driving habits, and type of roads/hwys you drive on, hopefully seeing the numbers will help clarify what the impact of any changes will be.........

**Sorry that the column headers don't line up with the data below, copied & pasted from excel spreadsheet, forum formatting makes the alignment go all wonky

PPS: You stated 235/70/15, but many trucks came with 235/75/15's, so if your tire size was a typo, it WILL make a difference, as the 75 series is close to an inch taller (235/70 @ ~27.95", 235/75 @ ~28.88")

James

 

I run 31" tires with 4.11s. The AOD basically turns them into 2.75s. 1750rpm is 55mph. 2000rpm is 65mph. 2500rpm is 80mph. Again, with 31" tall tires.

With a shorter tire, I'd probably reccomemd 3.70 or maybe 3.90 as mentioned. Each jump in gearing will bump you about 100-150 rpm.

 

And actually, yes, it does drop it almost 1000rpm. It's a steep OD ratio. .667:1.

 

I'd go with a 3.90....

 

The AOD has a 1st gear ratio of 2.40:1 and an overdrive ratio of .68:1.

With a 4.11 rear end ratio, the final drive equivalent, when in overdrive, would be 2.79:1 (.68 × 4.11 = 2.79).

Final drive equivalent with 3.70 rear end gears would be 2.52:1.

Final drive with 3.89 rear end gears would be 2.65:1.

3.91 rear end gears would be 2.66:1.

4.30 rear end gears would be 2.92:1.

4.57 rear gears would be 3.11:1.

For any vehicle to be a strong street performer, you need a power multiplication factor of at least 9:1. This is figured by multiplying the 1st gear ratio of the transmission by the rear end ratio.

2.40 x 4.11 = 9.86:1.

With a 4R70W transmission, the power multiplication factor with the same 4.11 rear gears would be 11.67:1 (2.84 x 4.11 = 11.67).

 

Wow thanks. It's awesome when the experts chime in. Lots of good info.

So I will go with either 3.90 or 4.11.

That's what makes this forum great. I can spend hours searching for answers and still confused. Then post a thread and BAM!, the data gets posted.

Thanks again,
Kenny
Also my truck is mostly street driving with some highway cruising

 

That's a big change. I had a F150 with AOD, but didn't remember it dropping that much. That's a big help on the highway! Mine runs almost 3200 to get 70mph with 3:70 gears and 30" tires.